2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[Inst]{The @Inst@ type: dictionaries or method instances}
8 LIE, emptyLIE, unitLIE, plusLIE, consLIE, zonkLIE,
9 plusLIEs, mkLIE, isEmptyLIE, lieToList, listToLIE,
12 pprInst, pprInsts, pprInstsInFull, tidyInsts, tidyMoreInsts,
14 newDictsFromOld, newDicts, cloneDict,
15 newMethod, newMethodWithGivenTy, newMethodAtLoc,
16 newOverloadedLit, newIPDict, tcInstCall, tcInstDataCon,
18 tyVarsOfInst, tyVarsOfInsts, tyVarsOfLIE,
19 ipNamesOfInst, ipNamesOfInsts, predsOfInst, predsOfInsts,
20 instLoc, getDictClassTys, dictPred,
22 lookupInst, lookupSimpleInst, LookupInstResult(..),
24 isDict, isClassDict, isMethod,
25 isLinearInst, linearInstType,
26 isTyVarDict, isStdClassTyVarDict, isMethodFor,
27 instBindingRequired, instCanBeGeneralised,
32 InstOrigin(..), InstLoc, pprInstLoc
35 #include "HsVersions.h"
37 import HsSyn ( HsLit(..), HsOverLit(..), HsExpr(..) )
38 import TcHsSyn ( TcExpr, TcId, TypecheckedHsExpr,
39 mkHsTyApp, mkHsDictApp, mkHsConApp, zonkId
42 import TcEnv ( TcIdSet, tcGetInstEnv, tcLookupId )
43 import InstEnv ( InstLookupResult(..), lookupInstEnv )
44 import TcMType ( zonkTcType, zonkTcTypes, zonkTcPredType, zapToType,
45 zonkTcThetaType, tcInstTyVar, tcInstType, tcInstTyVars
47 import TcType ( Type, TcType, TcThetaType, TcPredType, TcTauType, TcTyVarSet,
48 SourceType(..), PredType, ThetaType, TyVarDetails(VanillaTv),
49 tcSplitForAllTys, tcSplitForAllTys, mkTyConApp,
50 tcSplitMethodTy, tcSplitPhiTy, tcFunArgTy,
51 isIntTy,isFloatTy, isIntegerTy, isDoubleTy,
52 tcIsTyVarTy, mkPredTy, mkTyVarTy, mkTyVarTys,
53 tyVarsOfType, tyVarsOfTypes, tyVarsOfPred, tidyPred,
54 isClassPred, isTyVarClassPred, isLinearPred,
55 getClassPredTys, getClassPredTys_maybe, mkPredName,
56 tidyType, tidyTypes, tidyFreeTyVars,
57 tcCmpType, tcCmpTypes, tcCmpPred
59 import CoreFVs ( idFreeTyVars )
60 import Class ( Class )
61 import DataCon ( dataConSig )
62 import Id ( Id, idName, idType, mkUserLocal, mkSysLocal, mkLocalId, setIdUnique )
63 import PrelInfo ( isStandardClass, isCcallishClass, isNoDictClass )
64 import Name ( Name, mkMethodOcc, getOccName )
65 import PprType ( pprPred, pprParendType )
66 import Subst ( emptyInScopeSet, mkSubst,
67 substTy, substTyWith, substTheta, mkTyVarSubst, mkTopTyVarSubst
69 import Literal ( inIntRange )
70 import VarEnv ( TidyEnv, lookupSubstEnv, SubstResult(..) )
71 import VarSet ( elemVarSet, emptyVarSet, unionVarSet )
72 import TysWiredIn ( floatDataCon, doubleDataCon )
73 import PrelNames( fromIntegerName, fromRationalName )
74 import Util ( thenCmp, equalLength )
75 import BasicTypes( IPName(..), mapIPName, ipNameName )
81 %************************************************************************
83 \subsection[Inst-collections]{LIE: a collection of Insts}
85 %************************************************************************
90 isEmptyLIE = isEmptyBag
92 unitLIE inst = unitBag inst
93 mkLIE insts = listToBag insts
94 plusLIE lie1 lie2 = lie1 `unionBags` lie2
95 consLIE inst lie = inst `consBag` lie
96 plusLIEs lies = unionManyBags lies
100 zonkLIE :: LIE -> NF_TcM LIE
101 zonkLIE lie = mapBagNF_Tc zonkInst lie
103 pprInsts :: [Inst] -> SDoc
104 pprInsts insts = parens (sep (punctuate comma (map pprInst insts)))
108 = vcat (map go insts)
110 go inst = quotes (ppr inst) <+> pprInstLoc (instLoc inst)
113 %************************************************************************
115 \subsection[Inst-types]{@Inst@ types}
117 %************************************************************************
119 An @Inst@ is either a dictionary, an instance of an overloaded
120 literal, or an instance of an overloaded value. We call the latter a
121 ``method'' even though it may not correspond to a class operation.
122 For example, we might have an instance of the @double@ function at
123 type Int, represented by
125 Method 34 doubleId [Int] origin
137 TcId -- The overloaded function
138 -- This function will be a global, local, or ClassOpId;
139 -- inside instance decls (only) it can also be an InstId!
140 -- The id needn't be completely polymorphic.
141 -- You'll probably find its name (for documentation purposes)
142 -- inside the InstOrigin
144 [TcType] -- The types to which its polymorphic tyvars
145 -- should be instantiated.
146 -- These types must saturate the Id's foralls.
148 TcThetaType -- The (types of the) dictionaries to which the function
149 -- must be applied to get the method
151 TcTauType -- The type of the method
155 -- INVARIANT: in (Method u f tys theta tau loc)
156 -- type of (f tys dicts(from theta)) = tau
160 HsOverLit -- The literal from the occurrence site
161 TcType -- The type at which the literal is used
167 @Insts@ are ordered by their class/type info, rather than by their
168 unique. This allows the context-reduction mechanism to use standard finite
169 maps to do their stuff.
172 instance Ord Inst where
175 instance Eq Inst where
176 (==) i1 i2 = case i1 `cmpInst` i2 of
180 cmpInst (Dict _ pred1 _) (Dict _ pred2 _) = pred1 `tcCmpPred` pred2
181 cmpInst (Dict _ _ _) other = LT
183 cmpInst (Method _ _ _ _ _ _) (Dict _ _ _) = GT
184 cmpInst (Method _ id1 tys1 _ _ _) (Method _ id2 tys2 _ _ _) = (id1 `compare` id2) `thenCmp` (tys1 `tcCmpTypes` tys2)
185 cmpInst (Method _ _ _ _ _ _) other = LT
187 cmpInst (LitInst _ lit1 ty1 _) (LitInst _ lit2 ty2 _) = (lit1 `compare` lit2) `thenCmp` (ty1 `tcCmpType` ty2)
188 cmpInst (LitInst _ _ _ _) other = GT
190 -- and they can only have HsInt or HsFracs in them.
197 instName :: Inst -> Name
198 instName inst = idName (instToId inst)
200 instToId :: Inst -> TcId
201 instToId (Dict id _ _) = id
202 instToId (Method id _ _ _ _ _) = id
203 instToId (LitInst id _ _ _) = id
205 instLoc (Dict _ _ loc) = loc
206 instLoc (Method _ _ _ _ _ loc) = loc
207 instLoc (LitInst _ _ _ loc) = loc
209 dictPred (Dict _ pred _ ) = pred
210 dictPred inst = pprPanic "dictPred" (ppr inst)
212 getDictClassTys (Dict _ pred _) = getClassPredTys pred
214 predsOfInsts :: [Inst] -> [PredType]
215 predsOfInsts insts = concatMap predsOfInst insts
217 predsOfInst (Dict _ pred _) = [pred]
218 predsOfInst (Method _ _ _ theta _ _) = theta
219 predsOfInst (LitInst _ _ _ _) = []
220 -- The last case is is really a big cheat
221 -- LitInsts to give rise to a (Num a) or (Fractional a) predicate
222 -- But Num and Fractional have only one parameter and no functional
223 -- dependencies, so I think no caller of predsOfInst will care.
225 ipNamesOfInsts :: [Inst] -> [Name]
226 ipNamesOfInst :: Inst -> [Name]
227 -- Get the implicit parameters mentioned by these Insts
228 -- NB: ?x and %x get different Names
230 ipNamesOfInsts insts = [n | inst <- insts, n <- ipNamesOfInst inst]
232 ipNamesOfInst (Dict _ (IParam n _) _) = [ipNameName n]
233 ipNamesOfInst (Method _ _ _ theta _ _) = [ipNameName n | IParam n _ <- theta]
234 ipNamesOfInst other = []
236 tyVarsOfInst :: Inst -> TcTyVarSet
237 tyVarsOfInst (LitInst _ _ ty _) = tyVarsOfType ty
238 tyVarsOfInst (Dict _ pred _) = tyVarsOfPred pred
239 tyVarsOfInst (Method _ id tys _ _ _) = tyVarsOfTypes tys `unionVarSet` idFreeTyVars id
240 -- The id might have free type variables; in the case of
241 -- locally-overloaded class methods, for example
243 tyVarsOfInsts insts = foldr (unionVarSet . tyVarsOfInst) emptyVarSet insts
244 tyVarsOfLIE lie = tyVarsOfInsts (lieToList lie)
250 isDict :: Inst -> Bool
251 isDict (Dict _ _ _) = True
254 isClassDict :: Inst -> Bool
255 isClassDict (Dict _ pred _) = isClassPred pred
256 isClassDict other = False
258 isTyVarDict :: Inst -> Bool
259 isTyVarDict (Dict _ pred _) = isTyVarClassPred pred
260 isTyVarDict other = False
262 isMethod :: Inst -> Bool
263 isMethod (Method _ _ _ _ _ _) = True
264 isMethod other = False
266 isMethodFor :: TcIdSet -> Inst -> Bool
267 isMethodFor ids (Method uniq id tys _ _ loc) = id `elemVarSet` ids
268 isMethodFor ids inst = False
270 isLinearInst :: Inst -> Bool
271 isLinearInst (Dict _ pred _) = isLinearPred pred
272 isLinearInst other = False
273 -- We never build Method Insts that have
274 -- linear implicit paramters in them.
275 -- Hence no need to look for Methods
278 linearInstType :: Inst -> TcType -- %x::t --> t
279 linearInstType (Dict _ (IParam _ ty) _) = ty
282 isStdClassTyVarDict (Dict _ pred _) = case getClassPredTys_maybe pred of
283 Just (clas, [ty]) -> isStandardClass clas && tcIsTyVarTy ty
287 Two predicates which deal with the case where class constraints don't
288 necessarily result in bindings. The first tells whether an @Inst@
289 must be witnessed by an actual binding; the second tells whether an
290 @Inst@ can be generalised over.
293 instBindingRequired :: Inst -> Bool
294 instBindingRequired (Dict _ (ClassP clas _) _) = not (isNoDictClass clas)
295 instBindingRequired other = True
297 instCanBeGeneralised :: Inst -> Bool
298 instCanBeGeneralised (Dict _ (ClassP clas _) _) = not (isCcallishClass clas)
299 instCanBeGeneralised other = True
303 %************************************************************************
305 \subsection{Building dictionaries}
307 %************************************************************************
310 newDicts :: InstOrigin
314 = tcGetInstLoc orig `thenNF_Tc` \ loc ->
315 newDictsAtLoc loc theta
317 cloneDict :: Inst -> NF_TcM Inst
318 cloneDict (Dict id ty loc) = tcGetUnique `thenNF_Tc` \ uniq ->
319 returnNF_Tc (Dict (setIdUnique id uniq) ty loc)
321 newDictsFromOld :: Inst -> TcThetaType -> NF_TcM [Inst]
322 newDictsFromOld (Dict _ _ loc) theta = newDictsAtLoc loc theta
324 -- Local function, similar to newDicts,
325 -- but with slightly different interface
326 newDictsAtLoc :: InstLoc
329 newDictsAtLoc inst_loc@(_,loc,_) theta
330 = tcGetUniques `thenNF_Tc` \ new_uniqs ->
331 returnNF_Tc (zipWith mk_dict new_uniqs theta)
333 mk_dict uniq pred = Dict (mkLocalId (mkPredName uniq loc pred) (mkPredTy pred)) pred inst_loc
335 -- For vanilla implicit parameters, there is only one in scope
336 -- at any time, so we used to use the name of the implicit parameter itself
337 -- But with splittable implicit parameters there may be many in
338 -- scope, so we make up a new name.
339 newIPDict :: InstOrigin -> IPName Name -> Type
340 -> NF_TcM (IPName Id, Inst)
341 newIPDict orig ip_name ty
342 = tcGetInstLoc orig `thenNF_Tc` \ inst_loc@(_,loc,_) ->
343 tcGetUnique `thenNF_Tc` \ uniq ->
345 pred = IParam ip_name ty
346 id = mkLocalId (mkPredName uniq loc pred) (mkPredTy pred)
348 returnNF_Tc (mapIPName (\n -> id) ip_name, Dict id pred inst_loc)
352 %************************************************************************
354 \subsection{Building methods (calls of overloaded functions)}
356 %************************************************************************
360 tcInstCall :: InstOrigin -> TcType -> NF_TcM (TypecheckedHsExpr -> TypecheckedHsExpr, LIE, TcType)
361 tcInstCall orig fun_ty -- fun_ty is usually a sigma-type
362 = tcInstType VanillaTv fun_ty `thenNF_Tc` \ (tyvars, theta, tau) ->
363 newDicts orig theta `thenNF_Tc` \ dicts ->
365 inst_fn e = mkHsDictApp (mkHsTyApp e (mkTyVarTys tyvars)) (map instToId dicts)
367 returnNF_Tc (inst_fn, mkLIE dicts, tau)
369 tcInstDataCon orig data_con
371 (tvs, stupid_theta, ex_tvs, ex_theta, arg_tys, tycon) = dataConSig data_con
372 -- We generate constraints for the stupid theta even when
373 -- pattern matching (as the Report requires)
375 tcInstTyVars VanillaTv (ex_tvs ++ tvs) `thenNF_Tc` \ (all_tvs', ty_args', tenv) ->
377 stupid_theta' = substTheta tenv stupid_theta
378 ex_theta' = substTheta tenv ex_theta
379 arg_tys' = map (substTy tenv) arg_tys
381 n_ex_tvs = length ex_tvs
382 ex_tvs' = take n_ex_tvs all_tvs'
383 result_ty = mkTyConApp tycon (drop n_ex_tvs ty_args')
385 newDicts orig stupid_theta' `thenNF_Tc` \ stupid_dicts ->
386 newDicts orig ex_theta' `thenNF_Tc` \ ex_dicts ->
388 -- Note that we return the stupid theta *only* in the LIE;
389 -- we don't otherwise use it at all
390 returnNF_Tc (ty_args', map instToId ex_dicts, arg_tys', result_ty,
391 mkLIE stupid_dicts, mkLIE ex_dicts, ex_tvs')
394 newMethod :: InstOrigin
398 newMethod orig id tys
399 = -- Get the Id type and instantiate it at the specified types
401 (tyvars, rho) = tcSplitForAllTys (idType id)
402 rho_ty = substTyWith tyvars tys rho
403 (pred, tau) = tcSplitMethodTy rho_ty
405 newMethodWithGivenTy orig id tys [pred] tau
407 newMethodWithGivenTy orig id tys theta tau
408 = tcGetInstLoc orig `thenNF_Tc` \ loc ->
409 newMethodWith loc id tys theta tau
411 newMethodWith inst_loc@(_,loc,_) id tys theta tau
412 = tcGetUnique `thenNF_Tc` \ new_uniq ->
414 meth_id = mkUserLocal (mkMethodOcc (getOccName id)) new_uniq tau loc
416 returnNF_Tc (Method meth_id id tys theta tau inst_loc)
418 newMethodAtLoc :: InstLoc
420 -> NF_TcM (Inst, TcId)
421 newMethodAtLoc inst_loc real_id tys
422 -- This actually builds the Inst
423 = -- Get the Id type and instantiate it at the specified types
425 (tyvars,rho) = tcSplitForAllTys (idType real_id)
426 rho_ty = ASSERT( equalLength tyvars tys )
427 substTy (mkTopTyVarSubst tyvars tys) rho
428 (theta, tau) = tcSplitPhiTy rho_ty
430 newMethodWith inst_loc real_id tys theta tau `thenNF_Tc` \ meth_inst ->
431 returnNF_Tc (meth_inst, instToId meth_inst)
434 In newOverloadedLit we convert directly to an Int or Integer if we
435 know that's what we want. This may save some time, by not
436 temporarily generating overloaded literals, but it won't catch all
437 cases (the rest are caught in lookupInst).
440 newOverloadedLit :: InstOrigin
443 -> NF_TcM (TcExpr, LIE)
444 newOverloadedLit orig lit expected_ty
445 | Just expr <- shortCutLit lit expected_ty
446 = returnNF_Tc (expr, emptyLIE)
449 = tcGetInstLoc orig `thenNF_Tc` \ loc ->
450 tcGetUnique `thenNF_Tc` \ new_uniq ->
451 zapToType expected_ty `thenNF_Tc_`
452 -- The expected type might be a 'hole' type variable,
453 -- in which case we must zap it to an ordinary type variable
455 lit_inst = LitInst lit_id lit expected_ty loc
456 lit_id = mkSysLocal FSLIT("lit") new_uniq expected_ty
458 returnNF_Tc (HsVar (instToId lit_inst), unitLIE lit_inst)
460 shortCutLit :: HsOverLit -> TcType -> Maybe TcExpr
461 shortCutLit (HsIntegral i fi) ty
462 | isIntTy ty && inIntRange i && fi == fromIntegerName -- Short cut for Int
463 = Just (HsLit (HsInt i))
464 | isIntegerTy ty && fi == fromIntegerName -- Short cut for Integer
465 = Just (HsLit (HsInteger i))
467 shortCutLit (HsFractional f fr) ty
468 | isFloatTy ty && fr == fromRationalName
469 = Just (mkHsConApp floatDataCon [] [HsLit (HsFloatPrim f)])
470 | isDoubleTy ty && fr == fromRationalName
471 = Just (mkHsConApp doubleDataCon [] [HsLit (HsDoublePrim f)])
478 %************************************************************************
482 %************************************************************************
484 Zonking makes sure that the instance types are fully zonked,
485 but doesn't do the same for any of the Ids in an Inst. There's no
486 need, and it's a lot of extra work.
489 zonkInst :: Inst -> NF_TcM Inst
490 zonkInst (Dict id pred loc)
491 = zonkTcPredType pred `thenNF_Tc` \ new_pred ->
492 returnNF_Tc (Dict id new_pred loc)
494 zonkInst (Method m id tys theta tau loc)
495 = zonkId id `thenNF_Tc` \ new_id ->
496 -- Essential to zonk the id in case it's a local variable
497 -- Can't use zonkIdOcc because the id might itself be
498 -- an InstId, in which case it won't be in scope
500 zonkTcTypes tys `thenNF_Tc` \ new_tys ->
501 zonkTcThetaType theta `thenNF_Tc` \ new_theta ->
502 zonkTcType tau `thenNF_Tc` \ new_tau ->
503 returnNF_Tc (Method m new_id new_tys new_theta new_tau loc)
505 zonkInst (LitInst id lit ty loc)
506 = zonkTcType ty `thenNF_Tc` \ new_ty ->
507 returnNF_Tc (LitInst id lit new_ty loc)
509 zonkInsts insts = mapNF_Tc zonkInst insts
513 %************************************************************************
515 \subsection{Printing}
517 %************************************************************************
519 ToDo: improve these pretty-printing things. The ``origin'' is really only
520 relevant in error messages.
523 instance Outputable Inst where
524 ppr inst = pprInst inst
526 pprInst (LitInst u lit ty loc)
527 = hsep [ppr lit, ptext SLIT("at"), ppr ty, show_uniq u]
529 pprInst (Dict u pred loc) = pprPred pred <+> show_uniq u
531 pprInst m@(Method u id tys theta tau loc)
532 = hsep [ppr id, ptext SLIT("at"),
533 brackets (sep (map pprParendType tys)) {- ,
534 ptext SLIT("theta"), ppr theta,
535 ptext SLIT("tau"), ppr tau
539 show_uniq u = ifPprDebug (text "{-" <> ppr u <> text "-}")
541 tidyInst :: TidyEnv -> Inst -> Inst
542 tidyInst env (LitInst u lit ty loc) = LitInst u lit (tidyType env ty) loc
543 tidyInst env (Dict u pred loc) = Dict u (tidyPred env pred) loc
544 tidyInst env (Method u id tys theta tau loc) = Method u id (tidyTypes env tys) theta tau loc
546 tidyMoreInsts :: TidyEnv -> [Inst] -> (TidyEnv, [Inst])
547 -- This function doesn't assume that the tyvars are in scope
548 -- so it works like tidyOpenType, returning a TidyEnv
549 tidyMoreInsts env insts
550 = (env', map (tidyInst env') insts)
552 env' = tidyFreeTyVars env (tyVarsOfInsts insts)
554 tidyInsts :: [Inst] -> (TidyEnv, [Inst])
555 tidyInsts insts = tidyMoreInsts emptyTidyEnv insts
559 %************************************************************************
561 \subsection{Looking up Insts}
563 %************************************************************************
566 data LookupInstResult s
568 | SimpleInst TcExpr -- Just a variable, type application, or literal
569 | GenInst [Inst] TcExpr -- The expression and its needed insts
572 -> NF_TcM (LookupInstResult s)
576 lookupInst dict@(Dict _ (ClassP clas tys) loc)
577 = getDOptsTc `thenNF_Tc` \ dflags ->
578 tcGetInstEnv `thenNF_Tc` \ inst_env ->
579 case lookupInstEnv dflags inst_env clas tys of
581 FoundInst tenv dfun_id
582 -> -- It's possible that not all the tyvars are in
583 -- the substitution, tenv. For example:
584 -- instance C X a => D X where ...
585 -- (presumably there's a functional dependency in class C)
586 -- Hence the mk_ty_arg to instantiate any un-substituted tyvars.
588 (tyvars, rho) = tcSplitForAllTys (idType dfun_id)
589 mk_ty_arg tv = case lookupSubstEnv tenv tv of
590 Just (DoneTy ty) -> returnNF_Tc ty
591 Nothing -> tcInstTyVar VanillaTv tv `thenNF_Tc` \ tc_tv ->
592 returnTc (mkTyVarTy tc_tv)
594 mapNF_Tc mk_ty_arg tyvars `thenNF_Tc` \ ty_args ->
596 dfun_rho = substTy (mkTyVarSubst tyvars ty_args) rho
597 (theta, _) = tcSplitPhiTy dfun_rho
598 ty_app = mkHsTyApp (HsVar dfun_id) ty_args
601 returnNF_Tc (SimpleInst ty_app)
603 newDictsAtLoc loc theta `thenNF_Tc` \ dicts ->
605 rhs = mkHsDictApp ty_app (map instToId dicts)
607 returnNF_Tc (GenInst dicts rhs)
609 other -> returnNF_Tc NoInstance
611 lookupInst dict@(Dict _ _ loc) = returnNF_Tc NoInstance
615 lookupInst inst@(Method _ id tys theta _ loc)
616 = newDictsAtLoc loc theta `thenNF_Tc` \ dicts ->
617 returnNF_Tc (GenInst dicts (mkHsDictApp (mkHsTyApp (HsVar id) tys) (map instToId dicts)))
621 -- Look for short cuts first: if the literal is *definitely* a
622 -- int, integer, float or a double, generate the real thing here.
623 -- This is essential (see nofib/spectral/nucleic).
624 -- [Same shortcut as in newOverloadedLit, but we
625 -- may have done some unification by now]
627 lookupInst inst@(LitInst u lit ty loc)
628 | Just expr <- shortCutLit lit ty
629 = returnNF_Tc (GenInst [] expr) -- GenInst, not SimpleInst, because
630 -- expr may be a constructor application
632 lookupInst inst@(LitInst u (HsIntegral i from_integer_name) ty loc)
633 = tcLookupId from_integer_name `thenNF_Tc` \ from_integer ->
634 newMethodAtLoc loc from_integer [ty] `thenNF_Tc` \ (method_inst, method_id) ->
635 returnNF_Tc (GenInst [method_inst]
636 (HsApp (HsVar method_id) (HsLit (HsInteger i))))
639 lookupInst inst@(LitInst u (HsFractional f from_rat_name) ty loc)
640 = tcLookupId from_rat_name `thenNF_Tc` \ from_rational ->
641 newMethodAtLoc loc from_rational [ty] `thenNF_Tc` \ (method_inst, method_id) ->
643 rational_ty = tcFunArgTy (idType method_id)
644 rational_lit = HsLit (HsRat f rational_ty)
646 returnNF_Tc (GenInst [method_inst] (HsApp (HsVar method_id) rational_lit))
649 There is a second, simpler interface, when you want an instance of a
650 class at a given nullary type constructor. It just returns the
651 appropriate dictionary if it exists. It is used only when resolving
652 ambiguous dictionaries.
655 lookupSimpleInst :: Class
656 -> [Type] -- Look up (c,t)
657 -> NF_TcM (Maybe ThetaType) -- Here are the needed (c,t)s
659 lookupSimpleInst clas tys
660 = getDOptsTc `thenNF_Tc` \ dflags ->
661 tcGetInstEnv `thenNF_Tc` \ inst_env ->
662 case lookupInstEnv dflags inst_env clas tys of
664 -> returnNF_Tc (Just (substTheta (mkSubst emptyInScopeSet tenv) theta))
666 (_, rho) = tcSplitForAllTys (idType dfun)
667 (theta,_) = tcSplitPhiTy rho
669 other -> returnNF_Tc Nothing